In many applications in the field of recording and archival storage medium, e.g., direct-read-after-write archival storage medium for recording high density data and high quality images in particular, a life time for reliable data substantially longer than ten years is most desirable. It is also very desirable that phototools such as photomasks and reticles can be made substantially more durable than the chrome-phototools.
One way to achieve these objectives is to have the recorded image and/or the masking pattern and/or the information bearing layer present within (that is, not coated on) the glass substrate itself. It would improve the durability of the phototool and the recorded image, and at the same time eliminate the need of thin film coatings, such as the chromium film on a photomask and ablative thin film material, e.g. Tellurium film on optical disks.
There exist many types of glasses whereby optical images can be produced within a glass article. The merits and faults of each of the more promising families of glasses capable of storing optical image/data are discussed below.
U.S. Pat. No. 4,017,318 refers to photosensitive colored glasses which are the sodium halide-silver halide containing photosensitive glasses based on sodium silicate. The photosensitive colored glasses contain a significant amount of fluoride, 1-4% by weight of F, and a second halide usually about 1% by weight of Br. On cooling from melts, they become supersaturated with NaF. They also contain 0.0005-0.1% by weight of Ag as sensitizer and colorant, 0.01-0.2% CeO.sub.2 as optical sensitizer, about 1% Sb.sub.2 O.sub.3 and/or up to about 0.1% SnO as thermal sensitizer and redox agent. Through a unique sequence of ultraviolet exposures and heat treatments, a full spectrum of visible colors can be developed in a single piece of the alkali halide-silver halide containing photosensitive colored glasses of U.S. Pat. No. 4,017,318. While the intensity of colors depends on the exposure dosage of a second uv exposure, the hue is primarily a function of the first uv exposure dose. Thus, optical transmission curves for a fully developed glass show a progressive series of absorption bands moving from the infrared across the visible spectrum as a function of the increasing dosage of first ultraviolet exposures. The photosensitive colored glasses of U.S. Pat. No. 4,017,318 is often yellow colored in the regions/areas which are not exposed to ultraviolet radiation, after the sequence of heat treatments. In order to prevent the spontaneous yellow background coloration, the silver concentration in the photosensitive colored glass melts of U.S. Pat. No. 4,017,318 is kept below 0.025% by weight. As a result, the required thickness of the image bearing glass layer is at least about 100 micrometer to secure an optical density of unity. Thus the photosensitive colored glasses of U.S. Pat. No. 4,017,318 are not suitable for use as a recording medium to store data having bit sizes of less than about 10 micrometer and are not suitable for use as photomasks to print IC patterns having minimum geometries of less than about 10 micrometer.
U.S. Pat. No. 2,515,936 relates to silver-photosensitive glass, in which exposure to short wave radiation such as ultraviolet (310 nm) brings about a change. That is, irradiated areas are capable of heat developed coloration while non-irradiated areas remain substantially unchanged on heating. The heat developed image is permanent and is yellow to amber in color due to an absorption band centered at about 420 nm. The silver photosensitive glass of U.S. Pat. No. 2,515,936 refers specifically to glass batches on the weight percent basis equivalent to about 75% SiO.sub.2, 15% Alkali metal oxide and 10% divalent metal oxide, 0.05 to 0.3% silver computed as AgCl, and CeO.sub.2 in an amount up to 0.05% CeO.sub.2 is stated to act as a photo-sensitizing agent. Although up to 0.3% of silver can be retained in the glass melt, the thickness of the silver-photosensitive glass, required to secure 1 unit of optical density at 420 nm, is not less than that of the photosensitive colored glasses of U.S. Pat. No. 4,017,318. Apparently a smaller fraction of silver ions in the silver-photosensitive glass of U.S. Pat. No. 2,515,936 is reduced and precipitated out as colloidal silver metal particles in glass as a result of a radiation step followed by a heat treatment.
U.S. Pat. No. 2,732,298 discloses the method of producing a permanent two dimensional photographic image within the surface of a silicate glass containing an alkali metal oxide. In accordance with this method, there is first formed on the surface of a silicate glass by conventional procedures a chemically fixed photographic image. The glass, together with the image is then heated either in air alone or with an additional oxidizing agent present at a temperature between about 125.degree. C. below the strain point of the glass and just below its softening point for a time sufficient to oxidize and ionize the silver and to cause the migration of silver ions into the glass in exchange for alkali metal ions therein. A visible image which is usually of a yellowish to brownish color is produced due to the spontaneous reduction of a portion of the silver ions in the glass to metallic particles. If desired, a more complete reduction of silver ions and intensification of the color to dark amber or brown can be accomplished by subsequently heating the glass in a reducing atmosphere containing a reducing gas such as hydrogen, carbon monoxide or methane. Due to the lateral or sideway diffusion of the photographic silver image, the minimum geometry and/or line width reproducible in glass according to the method of U.S. Pat. No. 2,732,298 is in general greater than about twenty micrometers.
A modification of the method of U.S. Pat. No. 2,732,298 is found in U.S. Pat. No. 4,286,052 wherein the photographic silver image is driven into a glass article by electrical means.
Besides being not suitable for recording optical images or information with bit sizes or minimum geometries of less than a couple of micrometers, images in the three types of glasses cited above are developed through at least a heat treatment step, there is thus no direct-read-write capability.
U.S. Pat. Nos. 4,191,547 and 4,296,479, disclosed by the present author, refer to alterable photosensitive glasses, that is, photosensitive colored glasses exhibiting alterable photo-anisotropic effects. These glasses may be employed as erasable recording media having limited read applications. The alterable photosensitive glasses comprise a body portion and a photosensitive surface layer exhibiting alterable photo-anisotropic effects. The surface layer, having photosensitive and photo-anisotropic Ag--AgCl-containing crystals therein, is prepared by subjecting a glass article having base glass compositions within the Na.sub.2 O and/or K.sub.2 O--ZnO--Al.sub.2 O.sub.3 --SiO.sub.2 --Cl field to an ion exchange reaction in a silver containing aqueous solution, and subsequently exposing the glass article to radiation within the spectral ranges of uv and blue light. Write and erasure are done with polarized red light, and are based on the phenomena of photo-polarization and optical bleaching. The photo-anisotropic effects are inherent properties of the Ag--AgCl crystals within the surface layer of the alterable photosensitive glasses.
The alterable photosensitive glasses are not suitable for use as a permanent recording medium and/or as a phototool for the following obvious reasons. First, there exist no known method and/or mechanism of fixing the images. Thus the recorded image as well as the alterable photo-anisotropic surface layer are unstable in the presence of radiation in the wavelength range of uv, blue light and longer wavelengths, and are particularly sensitive to polarized red light. Second, the required thickness of the image/information bearing layer of the alterable photosensitive glasses is at least 30 micrometers to secure an optical density of 2.